Background:

• We have had a lot of arcing during hf transmissions.
  
• investigating under the dish while transmitting, we saw arcs on some of the vertical support posts around the center hole.
  
• When transmitting at 5 MHz, large arcs were induced in the 8 Mhz element on tower 1.

Covering the center of the dish:

    The large opening at the center of the bowl would not allow currents to flow in the center of the dish.  We decided to cover the hole by stringing cables (3/8 inch) across the hole in both directions. The spacing between the cables (in both directions) was made much less than the wavelength (5 MHz: 60meters, 8 MHz:37.5 meters).

On 17/18 jan17 we covered the open hole with  cables:

• we strung 5  cables in  the long direction of the hole .. connecting them to panels at each end.
• cables were strung along the short direction of the hole (every 5 feet?). They were also connected to the panel edges.
• Where the cables crossed, they were  tied together electrically  using ubolts.
• the cables were strung so that they did not touch the ao9 monument.
• The monument stuck up above the cables.
• the aluminum frame holding the ladders was below the cables.
• The western portion the opening was already covered with the long panels used to shield the receivers from the ground radiation.
• these long panels were only tied together at each end of the long direction.
  

The plots show the data we took while the hf was on (.ps) (.pdf)

• Each page has 3 frames:
• each color is a different transmitter (1 through 6).
  
• top  frame: rf power (in dbm) measured at the output of the rf coupler.
• middle frame: the same output power, but displayed in KWatts
• bottom frame:
• The synthesizer drive amplitude used
• The dashed green line shows when the rf was on
• the red line at the bottom is the total power from the carriage house 430 receiver (polA 425-435 mhz). When we have arcing, there are jumps in this power (rfi can also make jumps).
• Page 1 shows the data taken at 5 MHz
• tx5 failed around 12.0 hours..
  
• tx2 had a reflected power fault at 12.15 (when the output power was around 70KW). It was reset and then worked ok.
• Some of the jumps in the 430ch data (up to 12.52) could have been coming from tx5. It was running at reduced power while worked on it. After 12.52, its power was set to 0..
• Page 2 shows the data taken at 8 MHz.
• Tx 5 remained down.
• tx2 had more reflected power faults.
• we ran it at a lower power so alfredo could do some testing..
• We ended up running with 4 tx around 80 KWatts each.
• Tx4 had some troubles with its broad band amplifier.
  
• For awhile the output level was only about 50 Watts. The other TX's had outputs of over 100 Watts for the same drive level.
• after lowering and raising the power, tx4 started to work again.
• You can see that once it started working, it had a gradual drift down in output power over time (i wonder if we have a thermal issue?).

Summary:

• Covering the hole at the center of the dish:
• stopped the arcing on tower 1 8 mhz, when 5 MHz was transmitting.
• the arcing was way down. We ran at 80 KW with 5 tx (400 KW) and saw some (but not a lot) of arcing.
• So it looks like covering the center hole helped the arcing problem.
• We went under the dish while transmitting at 5 and 8 MHz.
• At 5 MHz
  
• we did not hear any arcing.
• I removed the strap on the veritcal pole (placed last nov) to see if it was still arcing... didn't see anything.
• At 8 MHz
• We heard no arcing
• We looked at the old arcing problem we had where ao9 connected to the earth.
  
• we removed the washer used to ground it.. and saw no arcing.
• The aluminum frame around ao9 no longer gave shocks when touched.
  
• Looking at the pole with the ground strap, i could still see tiny arcs if i brought the edge of the strap (sharp points) near the top part of the post. I then reinstalled the ground strap.
• We used the ultra sound detector to listen for arcs.
• No arcs were heard, but we could  hear rf pickup on the ultrasound amplifier.
• The pickup was nill when we sat under the new installed cables.
  
• The pickup was stronger when we moved to the long panels (not connected very well) on the west end of the hole.
• The pickup was also stronger as we walked under the normal reflector panels.
• I think this means that some 8 MHz is leaking through the dish. The individual panel connections are probably not perfect
• Tx 5 has problems.
• TX 2 was having reflected power faults, especially at 8 Mhz.
  
• The broad band amp on tx4 is not reliable.
• It can go to a mode where the output power jumps way down.. It later will come back.
• We need to verify that the input to the broad band is correct.
• We looked in the control room and the synth drive looked ok.
• We also looked at the spectra when the broadband output was low, we did not see large harmonics in the rfcoupler spectra (this could occur if the rf multiplexer was failing).

home_~phil